In a wireless communications system, the transmitter is used to transmit a wireless signal to the target receiver. When the power of wireless signal is larger, the transmission range is longer. Meanwhile, when the bandwidth of wireless signal is wider, the transmission rate is higher. However, the non-linear characteristic of the transmitter may cause some problems to the wireless system when the power and bandwidth of wireless signal are higher. For the example of LTE (Long Term Evolution) wireless system, a passive mixer is normally used to un-convert the wireless signal into an RF (Radio Frequency) signal, and a programmable gain amplifier or a power amplifier is used to power-up the RF signal for transmission. However, the non-linear characteristic of the passive mixer may introduce distortion around the frequency of the carrier frequency, i.e. the so called in-band distortion. Meanwhile, the passive mixer also generates harmonic signals at the 3rd harmonic frequency of the carrier frequency. Then, the non-linearity of the amplifier may fold the higher order harmonic signals back to the 1st order frequency (i.e. the carrier frequency) of the wireless signal, i.e. the so called CIM3 (Counter 3rd order intermodulation) folding back distortion. As a result, the power-up RF signal is distorted. Moreover, when the data rate of the wireless signal is reduced, e.g. only a partial RB (Resource Block) are used to transmit data, the CIM3 effect is more severe, and the CIM3 signal may affect the adjacent channels nearby the channel of the LTE wireless system. Therefore, how to reduce the effects caused by the non-linear characteristic of a wireless transmitter is an urgent problem in this field.